Correctly include C headers

This patch refactors VIXL to use `extern` block when including C header
that do not have a C++ counterpart.

Change-Id: I203d7d107755dbac3e5f4cf8d2f196f70dea1e07
diff --git a/src/code-buffer-vixl.h b/src/code-buffer-vixl.h
index 3b1d0d5..32aa4bc 100644
--- a/src/code-buffer-vixl.h
+++ b/src/code-buffer-vixl.h
@@ -27,7 +27,7 @@
 #ifndef VIXL_CODE_BUFFER_H
 #define VIXL_CODE_BUFFER_H
 
-#include <string.h>
+#include <cstring>
 
 #include "globals-vixl.h"
 #include "utils-vixl.h"